Liquid crystal display

ABSTRACT

To reduce viewing angle dependence of γ characteristics in a normally black liquid crystal display. 
     Each pixel  10  has a first sub-pixel  10   a  and a second sub-pixel  10   b  which can apply mutually different voltages to their respective liquid crystal layers. Relationships ΔV 12  (gk)&gt;0 volts and ΔV 12  (gk)≧ΔV 12  (gk+1) are satisfied at least in a range 0&lt;gk≦n−1 if it is assumed that ΔV 12 =V 1 −V 2 , where ΔV 12  is the difference between root-mean-square voltage V 1  applied to the liquid crystal layer of the first sub-pixel  10   a  and root-mean-square voltage V 2  applied to the liquid crystal layer of the second sub-pixel  10   b.

DESCRIPTION OF RELATED APPLICATIONS

The present application is a divisional of prior U.S. application Ser.No. 10/455,440 filed on Jun. 6, 2003, now U.S. Pat. No. 6,958,791 whichclaims priority under 35 U.S.C. § 119 to Japanese Application No.2002-165185 filed Jun. 6, 2002 and 2003-105334 filed Apr. 9, 2003, theentire contents of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a structure and/or drive method whichcan reduce viewing angle dependence of γ characteristics in a liquidcrystal display.

2. Description of the Related Art

Liquid crystal displays are flat-panel displays which have excellentfeatures including high resolution, small thickness, light weight, andlow power consumption. Their market size has expanded recently withimprovements in display performance and production capacity as well asimprovements in price competitiveness against other types of displaydevice.

Twisted nematic (TN) liquid crystal displays which have conventionallybeen in common use have liquid crystal molecules with positivedielectric anisotropy placed between upper and lower substrates in sucha way that their long axis are oriented approximately parallel tosubstrate surfaces and twisted 90 degrees along the thickness of aliquid crystal layer. When a voltage is applied to the liquid crystallayer, the liquid crystal molecules rise parallel to the electric field,releasing the twisted alignment. The TN liquid crystal display controlstransmitted light quantity using changes in rotary polarizationresulting from the orientation changes of the liquid crystal moleculescaused by voltage.

The TN liquid crystal display allows wide manufacturing margins and highproductivity. On the other hand, it has problems with displayperformance, especially with viewing angle characteristics.Specifically, when the display surface of the TN liquid crystal displayis viewed obliquely, the display contrast ratio lowers considerably.Consequently, even if an image clearly presents a plurality ofgrayscales from black to white when viewed from the front, brightnessdifferences between grayscales appear very unclear when the image isviewed obliquely. Besides, the phenomenon (so-called grayscale reversal)that a portion which appears dark when viewed from the front appearsbrighter when viewed obliquely also presents a problem.

To improve the viewing angle characteristics of the TN liquid crystaldisplay, some liquid crystal displays have been developed recently,including an in-plane switching (IPS) liquid crystal display describedin Japanese Patent Publication No. 63-21907, a multi-domain verticallyaligned (MVA) liquid crystal display described in Japanese PatentLaid-Open No. 11-242225, an Axial Symmetric Micro-cell (ASM) displaydescribed in Japanese Patent Laid-Open No. 10-186330, and a liquidcrystal display described in Japanese Patent Laid-Open No. 2002-55343.

Liquid crystal displays employing any of the novel modes described above(wide viewing angle modes) solve the concrete problems with viewingangle characteristics. Specifically they are free of the problems thatthe display contrast ratio lowers considerably or display grayscales arereversed when the display surface of the TN liquid crystal display isviewed obliquely.

Under circumstances where display quality of liquid crystal displayscontinues to be improved, a new problem with viewing anglecharacteristics have surfaced, namely, viewing angle dependence of γcharacteristics, meaning that γ characteristics differ between when thedisplay is viewed from the front and when the display is viewedobliquely. This presents a problem, especially when displaying imagessuch as photographs or displaying television broadcasts and the like.

The viewing angle dependence of γ characteristics is more prominent inMVA mode and ASM mode than in IPS mode. On the other hand, it is moredifficult to produce IPS panels which provide a high contrast ratio whenviewed from the front with high productivity than MVA or ASM panels.Thus, it is desired to reduce the viewing angle dependence of γcharacteristics in MVA mode or ASM mode.

The present invention has been made in view of the above points. Itsmain object is to provide a liquid crystal display with reduced viewingangle dependence of γ characteristics.

SUMMARY OF THE INVENTION

To achieve the above object, a first aspect of the present inventionprovides a liquid crystal display used in normally black mode,comprising a plurality of pixels each of which has a liquid crystallayer and a plurality of electrodes for applying voltage to the liquidcrystal layer, wherein: each of the plurality of pixels comprises afirst sub-pixel and a second sub-pixel which can apply mutuallydifferent voltages to their respective liquid crystal layers; and wheneach of the plurality of pixels displays a grayscale gk which satisfies0≦gk≦n, where gk and n are integers not less than zero and a largervalue of gk corresponds to higher brightness, relationships ΔV12 (gk)>0volts and ΔV12 (gk)≧ΔV12 (gk+1) are satisfied at least in a range0<gk≦n−1 if it is assumed that ΔV12 (gk)=V1 (gk)−V2 (gk), where V1 (gk)and V2 (gk) are root-mean-square voltages applied to the liquid crystallayers of the first sub-pixel and the second sub-pixel, respectively.

The liquid crystal display may be configured such that: each of theplurality of pixels comprises a third sub-pixel which can apply avoltage different from those of the first sub-pixel and the secondsub-pixel to its liquid crystal layer; and when each of the plurality ofpixels displays a grayscale gk and ΔV13 (gk)=V1 (gk)−V3 (gk), arelationship 0 volts<ΔV13 (gk)<ΔV12 (gk) is satisfied if theroot-mean-square voltage applied to the liquid crystal layer of thethird sub-pixel is V3 (gk).

Preferably, the root-mean-square voltages applied to the liquid crystallayers satisfy a relationship ΔV12 (gk)≧ΔV12 (gk+1) at least in a range0<gk≦n−1.

Preferably, relationships ΔV12 (gk)≧ΔV12 (gk+1) and ΔV13 (gk)≧ΔV13(gk+1) are satisfied at least in a range 0<gk≦n−1 when each pixel has athird sub-pixel.

In a preferred embodiment, the first sub-pixel and the second sub-pixeleach comprise: a liquid crystal capacitor formed by a counter electrodeand a sub-pixel electrode opposing the counter electrode via the liquidcrystal layer, and a storage capacitor formed by a storage capacitorelectrode connected electrically to the sub-pixel electrode, aninsulating layer, and a storage capacitor counter electrode opposing thestorage capacitor electrode via the insulating layer; and the counterelectrode is a single electrode shared by the first sub-pixel and thesecond sub-pixel, and the storage capacitor counter electrodes of thefirst sub-pixel and the second sub-pixel are electrically independent ofeach other. Typically, the counter electrode is provided on a countersubstrate (sometimes referred to as a “common electrode”), but in IPSmode, it is provided on the same substrate as the sub-pixel electrode.Incidentally, “the counter electrode opposing a sub-pixel electrode viathe liquid crystal layer” need not necessarily oppose the sub-pixelelectrode across the thickness of the liquid crystal layer. In an IPSliquid crystal display, it is placed within the liquid crystal layer inopposing relation to the sub-pixel electrode across the liquid crystallayer.

In a preferred embodiment, the liquid crystal display comprises twoswitching elements provided for the first sub-pixel and the secondsub-pixel, respectively, wherein the two switching elements are turnedon and off by scan line signal voltages supplied to a common scan line;display signal voltages are applied to the respective sub-pixelelectrodes and storage capacitor electrodes of the first sub-pixel andthe second sub-pixel from a common signal line when the two switchingelements are on; voltages of the respective storage capacitor counterelectrodes of the first sub-pixel and the second sub-pixel change afterthe two switching elements are turned off; and the amounts of changedefined by the direction and magnitude of the change differ between thefirst sub-pixel and the second sub-pixel. The amounts of change in thestorage capacitor counter electrodes are defined here not only in termsof magnitude (absolute value), but also in terms of direction. Forexample, the amounts of change in the voltages of the storage capacitorcounter electrodes of the first sub-pixel and the second sub-pixel maybe equal in absolute value and differ in sign. In short, if voltagerises in one of the storage capacitor counter electrodes and falls inthe other storage capacitor counter electrode after the switchingelement is turned off, the absolute values of the changes may be equal.

Preferably, the liquid crystal layer is a vertically aligned liquidcrystal layer and contains nematic liquid crystal material with negativedielectric anisotropy.

Preferably, the liquid crystal layers of the first sub-pixel and thesecond sub-pixel each contain four domains which are approximately 90degrees apart in azimuth direction in which their liquid crystalmolecules incline when a voltage is applied.

Preferably, the first sub-pixel and the second sub-pixel are placed onopposite sides of the common signal line; the first sub-pixel and thesecond sub-pixel each have, on the counter electrode side, a pluralityof ribs protruding towards the liquid crystal layer and the plurality ofribs include a first rib extending in a first direction and a second ribextending in a second direction approximately orthogonal to the firstdirection; and the first rib and the second rib are placed symmetricallywith respect to a center line parallel to the common scan line in eachof the first sub-pixel and the second sub-pixel and the arrangement ofthe first rib and the second rib in one of the first and secondsub-pixels is symmetrical with respect to the arrangement of the firstrib and the second rib in the other sub-pixel.

Preferably, the center line parallel to the common scan line in each ofthe first sub-pixel and the second sub-pixel is placed at an intervalequal to approximately one half of an array pitch of the scan lines inboth the first sub-pixel and the second sub-pixel.

Preferably, the area of the first sub-pixel is equal to or smaller thanthe area of the second sub-pixel. When each of the plurality of pixelshas three or more sub-pixels, preferably the area of the sub-pixel towhich the highest root-mean-square voltage is applied is not larger thanthe areas of the other sub-pixels.

In a liquid crystal display according to another aspect of the presentinvention: direction of the electric field applied to the liquid crystallayers in the plurality of pixels is reversed every vertical scanningperiod; and when displaying an intermediate grayscale, the direction ofthe electric field is reversed periodically in the row direction in thecase of pixels in an arbitrary row and it is reversed every pixel in thecolumn direction in the case of pixels in an arbitrary column.

According to one embodiment, the direction of the electric field isreversed every pixel in the row direction in the case of pixels in anarbitrary row.

According to one embodiment, the direction of the electric field isreversed every two pixels in the row direction in the case of pixels inan arbitrary row.

A liquid crystal display according to one embodiment, operates innormally black mode; wherein the at least two sub-pixels include twosub-pixels SPa (p, q) and SPb (p, q); and when each of the plurality ofpixels displays a grayscale gk which satisfies 0≦gk≦n, where gk and nare integers not less than zero and a larger value of gk corresponds tohigher brightness, relationships ΔV12 (gk)>0 volts and ΔV12 (gk)≧ΔV12(gk+1) are satisfied at least in a range 0<gk≦n−1 if it is assumed thatΔV12 (gk)=V1 (gk)−V2 (gk), where V1 (gk) and V2 (gk) areroot-mean-square voltages applied to the liquid crystal layers of thefirst sub-pixel and the second sub-pixel, respectively.

According to one embodiment, a relationship ΔV12 (gk)≧ΔV12 (gk+1) issatisfied at least in a range 0<gk≦n−1.

According to one embodiment, SPa (p, q) and SPb (p, q) each comprise: aliquid crystal capacitor formed by a counter electrode and a sub-pixelelectrode opposing the counter electrode via the liquid crystal layer,and a storage capacitor formed by a storage capacitor electrodeconnected electrically to the sub-pixel electrode, an insulating layer,and a storage capacitor counter electrode opposing the storage capacitorelectrode via the insulating layer; and the counter electrode is asingle electrode shared by SPa (p, q) and SPb (p, q), and the storagecapacitor counter electrodes of SPa (p, q) and SPb (p, q) areelectrically independent of each other.

According to one embodiment, the liquid crystal display comprises twoswitching elements provided for SPa (p, q) and SPb (p, q), respectively,wherein the two switching elements are turned on and off by scan linesignal voltages supplied to a common scan line; display signal voltagesare applied to the respective sub-pixel electrodes and storage capacitorelectrodes of SPa (p, q) and SPb (p, q) from a common signal line whenthe two switching elements are on; voltages of the respective storagecapacitor counter electrodes of SPa (p, q) and SPb (p, q) change afterthe two switching elements are turned off; and the amounts of changedefined by the direction and magnitude of the change differ between SPa(p, q) and SPb (p, q). Specifically, when the two switching elements areon, voltages are applied to the respective storage capacitor counterelectrodes of VSpa (on) and VSpb (on) such that when the two switchingelements are turned off, potentials of the respective storage capacitorcounter electrodes will change, for example, from VSpa (on) and VSpb(on) to VSpa (off) and VSpb (off), respectively, and that the respectiveamounts of change “VSpa (off)−VSpa (on)” and “VSpb (off)−VSpb (on)” willbe mutually different.

According to one embodiment, the changes in the voltages of the storagecapacitor counter electrodes of SPa (p, q) and SPb (p, q) are equal inamount and opposite in direction.

According to one embodiment, the voltages of the storage capacitorcounter electrodes of SPa (p, q) and SPb (p, q) are oscillating voltages180 degrees out of phase with each other. The oscillating voltages maybe rectangular waves, sine waves, or triangular waves.

According to one embodiment, the oscillating voltages of the storagecapacitor counter electrodes of SPa (p, q) and SPb (p, q) each have aperiod approximately equal to one horizontal scanning period.

According to one embodiment, the oscillating voltages of the storagecapacitor counter electrodes of SPa (p, q) and SPb (p, q) each have aperiod shorter than one horizontal scanning period.

According to one embodiment, the oscillating voltages of the storagecapacitor counter electrodes of SPa (p, q) and SPb (p, q) areapproximately equal within any horizontal scanning period if averagedover the period.

According to one embodiment, the period of the oscillation is one-halfof one horizontal scanning period.

According to one embodiment, the oscillating voltages are rectangularwaves with a duty ratio of 1:1.

According to one embodiment, SPa (p, q) and SPb (p, q) have differentareas, of which the smaller area belongs to SPa (p, q) or SPb (p, q)whichever has a larger root-mean-square voltage applied to its liquidcrystal layer.

According to one embodiment, the area of SPa (p, q) and area of SPb (p,q) are practically equal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing a pixel configuration of aliquid crystal display 100 according to an embodiment in a first aspectof the present invention.

FIGS. 2A to 2C are schematic diagrams showing a structure of a liquidcrystal display according to the embodiment of the present invention.

FIGS. 3A to 3C are diagrams schematically showing a structure of aconventional liquid crystal display 100′.

FIGS. 4A to 4C are diagrams illustrating display characteristics of anMVA liquid crystal display, where FIG. 4A is a graph showing dependenceof transmittance on applied voltage, FIG. 4B is a diagram showingtransmittances in FIG. 4A after being normalized with respect totransmittance in white mode, and FIG. 4C is a diagram showing γcharacteristics.

FIGS. 5A to 5D are diagrams showing conditions A to D, respectively, ofvoltages to be applied to liquid crystal layers of sub-pixels obtainedby dividing pixels.

FIGS. 6A to 6B are graphs showing γ characteristics obtained undervoltage conditions A to D, shown in FIG. 5, where FIG. 6A shows rightside 60-degree viewing γ characteristics and FIG. 6B shows upper-rightside 60-degree viewing γ characteristics.

FIG. 7 is a graph showing white-mode transmittance (frontal viewing)obtained under voltage conditions A to D.

FIGS. 8A to 8B are graphs illustrating effects of area ratios betweensub-pixels on γ characteristics under voltage condition C according tothe embodiment of the present invention, where FIG. 8A shows right side60-degree viewing γ characteristics and FIG. 6B shows upper-right side60-degree viewing γ characteristics.

FIG. 9 is a diagram showing relationship between white-modetransmittance (frontal viewing) and sub-pixel area ratios under voltagecondition C according to the embodiment of the present invention.

FIGS. 10A to 10B are diagrams illustrating effects of sub-pixel countson γ characteristics under voltage condition B according to theembodiment of the present invention, where FIG. 10A shows right side60-degree viewing γ characteristics and FIG. 10B shows upper-right side60-degree viewing γ characteristics.

FIG. 11 is a diagram showing relationship between white-modetransmittance (frontal viewing) and sub-pixel counts under voltagecondition B according to the embodiment of the present invention.

FIG. 12 is a schematic diagram showing a pixel structure of a liquidcrystal display 200 according to another embodiment of the presentinvention.

FIG. 13 is a diagram showing an equivalent circuit for a pixel of theliquid crystal display 200.

FIG. 14 is a diagram showing various voltage waveforms (a)–(f) fordriving the liquid crystal display 200.

FIG. 15 is a diagram showing relationship between voltages applied toliquid crystal layers of sub-pixels in the liquid crystal display 200.

FIGS. 16A to 16B are diagrams showing γ characteristics of the liquidcrystal display 200, where FIG. 16A shows right side 60-degree viewing γcharacteristics and FIG. 16B shows upper-right side 60-degree viewing γcharacteristics.

FIG. 17 is a diagram schematically showing a pixel arrangement of aliquid crystal display according to a second aspect of the presentinvention.

FIG. 18 is a diagram showing waveforms (a)–(j) of various voltages(signals) for driving the liquid crystal display which has theconfiguration shown in FIG. 17.

FIG. 19 is a diagram schematically showing a pixel arrangement of aliquid crystal display according to another embodiment of the presentinvention.

FIG. 20 is a diagram showing waveforms (a)–(j) of various voltages(signals) for driving the liquid crystal display which has theconfiguration shown in FIG. 19.

FIG. 21A is a diagram schematically showing a pixel arrangement of aliquid crystal display according to another embodiment of the presentinvention and FIG. 21B is a diagram schematically showing an arrangementof its storage capacitor lines and storage capacitor electrodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Configuration and operation of liquid crystal displays according toembodiments in a first aspect of the present invention will be describedbelow with reference to drawings.

First, refer to FIGS. 1, 2A, 2B, and 2C. FIG. 1 is a diagramschematically showing an electrode arrangement in a pixel of a liquidcrystal display 100 according to an embodiment of the present invention.FIG. 2A is a diagram schematically showing an overall configuration ofthe liquid crystal display 100, FIG. 2B is a diagram schematicallyshowing an electrode structure in a pixel, FIG. 2C is a sectional viewtaken along a line 2C–2C′ in FIG. 2B. For the purpose of reference, anelectrode arrangement in a pixel of a conventional liquid crystaldisplay 100′, its electrode structure, and a sectional view taken alonga line 3C–3C′ are shown schematically in FIGS. 3A, 3B, and 3C,respectively.

The liquid crystal display 100 according to this embodiment operates innormally black mode and comprises a plurality of pixels each of whichhas a liquid crystal layer and a plurality of electrodes for applyingvoltage to the liquid crystal layer. Although a TFT liquid crystaldisplay is taken as an example here, other switching elements (e.g., MIMelements) may be used instead.

The liquid crystal display 100 has a plurality of pixels 10 arranged ina matrix. Each of the plurality of pixels 10 has a liquid crystal layer13. Also, the pixels have their own pixel electrode 18 and a counterelectrode 17 to apply voltage to the liquid crystal layer 13. Typically,the counter electrode 17 is a single electrode common to all the pixels10.

In the liquid crystal display 100 according to this embodiment, each ofthe plurality of pixels 10 has a first sub-pixel 10 a and secondsub-pixel 10 b which can apply mutually different voltages, as shown inFIG. 1.

When displaying a grayscale gk which satisfies 0≦gk≦n (where gk and nare integers not less than zero and a larger value of gk corresponds tohigher brightness), each of the plurality of pixels is driven in such away as to satisfy relationships ΔV12 (gk)>0 volts and ΔV12 (gk)≧ΔV12(gk+1) at least in a range 0<gk≦n−1, where ΔV12 (gk)=V1 (gk)−V2 (gk) isthe difference between root-mean-square voltage V1 (gk) applied to theliquid crystal layer of the first sub-pixel 10 a and root-mean-squarevoltage V2 (gk) applied to the liquid crystal layer of the secondsub-pixel 10 b.

The number of sub-pixels (sometimes referred to as the number of pixeldivisions) possessed by each pixel 10 it not limited to two. Each pixel10 may further have a third sub-pixel (not shown) to which a voltagedifferent from those applied to the first sub-pixel 10 a and secondsub-pixel 10 b may be applied. In that case, the pixel is configuredsuch that a relationship 0 volts<ΔV13 (gk)<ΔV12 (gk) is satisfied if itis assumed ΔV13 (gk)=V1 (gk)−V3 (gk), where V3 (gk) is anroot-mean-square voltage applied to the liquid crystal layer of thethird sub-pixel and ΔV13 (gk) is the difference between theroot-mean-square voltage applied to the liquid crystal layer of thefirst sub-pixel and the root-mean-square voltage applied to the liquidcrystal layer of the third sub-pixel. Of course, each pixel 10 may havefour or more sub-pixels.

Preferably, the root-mean-square voltages applied to the liquid crystallayers of the sub-pixels satisfy a relationship ΔV12 (gk)>ΔV12 (gk+1) atleast in a range 0<gk≦n−1. Thus, it is preferable that as the grayscalelevel gets higher, the difference between the root-mean-square voltagesapplied to the liquid crystal layers of the first sub-pixel 10 a andsecond sub-pixel 10 b becomes smaller. In other words, it is preferablethat as the grayscale level gets lower (closer to black), the differencebetween the root-mean-square voltages applied to the liquid crystallayers of the first sub-pixel 10 a and second sub-pixel 10 b becomeslarger. Also, preferably relationships ΔV12 (gk)>ΔV12 (gk+1) and ΔV13(gk)>ΔV13 (gk+1) are satisfied at least in a range 0<gk≦n−1 if eachpixel has a third sub-pixel.

Preferably, the area of the first sub-pixel 10 a is equal to or smallerthan the area of the second sub-pixel 10 b. If each of the plurality ofpixels has three or more sub-pixels, preferably the area of thesub-pixel (the first sub-pixel in this case) to which the highestroot-mean-square voltage is applied is not larger than the area of thesub-pixel (the second sub-pixel in this case) to which the lowestroot-mean-square voltage is applied. Specifically, if each pixel 10 hasa plurality of sub-pixels SP1, SP2, . . . , and SPn and theroot-mean-square voltages applied to the liquid crystal layers are V1(gk), V2 (gk), . . . , and Vn (gk), preferably a relationship V1 (gk)>V2(gk)> . . . >Vn (gk) is satisfied. Also, if the areas of the sub-pixelsare SSP1, SSP2, . . . , and SSPn, preferably a relationship SSP1≦SSP2≦ .. . ≦SSPn is satisfied.

Effects of the present invention can be achieved, at least if therelationship V1 (gk),>V2 (gk)> . . . >Vn (gk) is satisfied for allgrayscales except the highest and lowest grayscales (i.e., in the range0<gk≦n−1). However, it is also possible to implement a configuration inwhich the relationship is satisfied for all the grayscales (i.e., in therange 0≦gk≦n).

In this way, if each pixel is divided into a plurality of sub-pixels anddifferent voltages are applied to the liquid crystal layers of thesub-pixels, a mixture of different γ characteristics are observed and,thus, the viewing angle dependence of γ characteristics is reduced.Furthermore, since the difference between root-mean-square voltages areset larger at lower grayscales, the viewing angle dependence of γcharacteristics is reduced greatly on the black side (at low brightnesslevels) in normally black mode. This is highly effective in improvingdisplay quality.

Various configurations are available to apply root-mean-square voltagesto the liquid crystal layers of the sub-pixels 10 a and 10 b in such away as to satisfy the above relationships.

For example, the liquid crystal display 100 can be configured as shownin FIG. 1. Specifically, whereas in the conventional liquid crystaldisplay 100′, a pixel 10 has only one pixel electrode 18 that isconnected to a signal line 14 via a TFT 16, the liquid crystal display100 has two sub-pixel electrodes 18 a and 18 b which are connected todifferent signal lines 14 a and 14 b via respective TFTs 16 a and 16 b.

Since the sub-pixels 10 a and 10 b compose one pixel 10, gates of theTFTs 16 a and 16 b are connected to a common scan line (gate busline) 12and turned on and off by a common scan signal. Signal voltages(grayscale voltages) which satisfy the above relationship are suppliedto signal lines (source busline) 14 a and 14 b. Preferably, the gates ofthe TFTs 16 a and 16 b are configured as a common gate.

Alternatively, in a configuration (described later) in which the firstsub-pixel and second sub-pixel each comprise storage capacitor which isformed by a storage capacitor electrode connected electrically to asub-pixel electrode, an insulating layer, and a storage capacitorcounter electrode opposing the storage capacitor electrode via theinsulating layer, it is preferable to provide the storage capacitorcounter electrodes of the first sub-pixel and second sub-pixel beingelectrically independent of each other, and vary the root-mean-squarevoltage applied to the liquid crystal layer of the first sub-pixel androot-mean-square voltage applied to the liquid crystal layer of secondsub-pixel using capacitance division by varying voltages (referred to asstorage capacitor counter electrode voltages) supplied to the storagecapacitor counter electrodes. By regulating the value of the storagecapacitor and magnitude of the voltages supplied to the storagecapacitor counter electrodes, it is possible to control the magnitudesof the root-mean-square voltages applied to the liquid crystal layers ofthe sub-pixels.

In this configuration, since there is no need to apply different signalvoltages to sub-pixel electrodes 18 a and 18 b, the TFTs 16 a and 16 bcan be connected to a common signal line and the same signal voltage canbe supplied to them. Therefore, the number of signal lines is the sameas in the case of the conventional liquid crystal display 100′ shown inFIG. 3 and it is possible to use a signal line drive circuit with thesame configuration as the conventional liquid crystal display 100′. Ofcourse, since the TFTs 16 a and 16 b are connected to the same scanline, preferably they share a common gate as in the case of the aboveexample.

Preferably, the present invention is applied to liquid crystal displayswhich use a vertically aligned liquid crystal layer containing nematicliquid crystal material with negative dielectric anisotropy. Inparticular, it is preferable that the liquid crystal layer of eachsub-pixel contains four domains which are approximately 90 degrees apartin azimuth direction in which their liquid crystal molecules inclinewhen a voltage is applied (MVA). Alternatively, the liquid crystal layerof each sub-pixel may maintain an axially symmetrical alignment at leastwhen voltage is applied (ASM).

The embodiment of the present invention will be described in more detailbelow in relation to an MVA liquid crystal display 100 in which theliquid crystal layer of each sub-pixel contains four domains which areapproximately 90 degrees apart in azimuth direction in which theirliquid crystal molecules incline when a voltage is applied.

As shown schematically in FIG. 2A, the MVA liquid crystal display 100comprises a liquid crystal panel 10A, phase difference compensatingelements (typically, phase difference compensating plates) 20 a and 20 bmounted on both sides of the liquid crystal panel 10A, polarizing plates30 a and 30 b which sandwich them, and a backlight 40. The transmissionaxes (also known as polarization axes) of the polarizing plates 30 a and30 b are orthogonal to each other (crossed-Nicols arrangement) so thatblack is displayed when no voltage is applied to the liquid crystallayer (not shown) of the liquid crystal panel 10A (in a state ofvertical alignment). The phase difference compensating elements 20 a and20 b are provided to improve viewing angle characteristics of the liquidcrystal display and are designed optimally using known technologies.Specifically, they have been optimized (gk=0) to minimize brightness(black level) differences between when a black screen is viewed from thefront and when it is viewed obliquely from any azimuth direction. Whenthe phase difference compensating elements 20 a and 20 b are optimizedin this way, the present invention can produce more marked effects.

As a matter of course, the common scan line 12, signal lines 14 a and 14b, and TFTs 16 a and 16 b (see FIG. 1) are formed on a substrate 11 a toapply predetermined signal voltages to the sub-pixel electrodes 18 a and18 b respectively at predetermined times. Also, to drive thesecomponents, circuits and the like are formed, as required. Besides,color filters and the like are provided on another substrate 11 b, asrequired.

Structure of a pixel in the MVA liquid crystal display 100 will bedescribed with reference to FIGS. 2A and 2C. Basic configuration andoperation of an MVA liquid crystal display is described, for example, inJapanese Patent Laid-Open No. 11-242225.

As described with reference to FIG. 1, the pixel 10 in the MVA liquidcrystal display 100 has two sub-pixels 10 a and 10 b, of which thesub-pixel 10 a has the sub-pixel electrode 18 a and the sub-pixel 10 bhas the sub-pixel electrode 18 b. As shown schematically in FIG. 2C, thesub-pixel electrode 18 a (and the sub-pixel electrode 18 b (not shown))formed on the glass substrate 11 a has a slit 18 s and forms a tiltedelectric field in conjunction with the counter electrode 17 which isplaced in opposing relation to the sub-pixel electrode 18 a across aliquid crystal layer 13. Also, ribs 19 protruding towards the liquidcrystal layer 13 are provided on a surface of the glass substrate 11 bon which the counter electrode 17 is mounted. The liquid crystal layer13 is made of nematic liquid crystal material with negative dielectricanisotropy. When no voltage is applied, it is aligned nearly verticallyby a vertical alignment film (not shown) which covers the counterelectrode 17, ribs 19, and sub-pixel electrodes 18 a and 18 b. Theliquid crystal molecules aligned vertically can be laid down safely in apredetermined direction by rib 19 surfaces (inclined faces) and thetilted electric field.

As shown in FIG. 2C, the rib 19 is inclined toward its center in such away as to form an angle. The liquid crystal molecules are aligned nearlyvertically to the inclined faces. Thus, the ribs 19 determinedistribution of the tilt angle (angle formed by the substrate surfaceand long axis of the liquid crystal molecules) of the liquid crystalmolecules. The slit 18 s regularly changes the direction of the electricfield applied to the liquid crystal layer. Consequently, when theelectric field is applied, the liquid crystal molecules are aligned bythe ribs 19 and slit 18 s in four directions—upper right, upper left,lower left, and lower right—indicated by arrows in the figure, providingvertically and horizontally symmetrical, good viewing anglecharacteristics. A rectangular display surface of the liquid crystalpanel 10A is typically oriented with its longer dimension placedhorizontally and the transmission axis of the polarizing plate 30 aplaced parallel to the longer dimension. On the other hand, the pixel 10is typically oriented with its longer dimension orthogonal to the longerdimension of the liquid crystal panel 10A as shown in FIG. 2B.

Preferably, as shown in FIG. 2B, the areas of the first sub-pixel 10 aand second sub-pixel 10 b are practically equal, each of the sub-pixelscontain a first rib extending in a first direction and a second ribextending in a second direction, the first rib and the second rib ineach sub-pixel are placed symmetrically with respect to a center lineparallel to the scan line 12, and rib arrangement in one of thesub-pixels and rib arrangement in the other sub-pixel are symmetricalwith respect to the center line orthogonal to the scan line 12. Thisarrangement causes the liquid crystal molecules in each sub-pixel to bealigned in four directions—upper right, upper left, lower left, andlower right—and makes the areas of the liquid crystal domains in theentire pixel including the first sub-pixel and second sub-pixelpractically equal, providing vertically and horizontally symmetrical,good viewing angle characteristics. This effect is prominent when thearea of the pixel is small. Furthermore, it is preferable that thecenter line parallel to the common scan line in each sub-pixel is placedat an interval equal to approximately one half of an array pitch of thescan line.

Next, description will be given of operation and display characteristicsof the liquid crystal display 100 according to the embodiment of thepresent invention.

First, with reference to FIG. 4, description will be given of displaycharacteristics of the MVA liquid crystal display which has the sameelectrode configuration as the conventional liquid crystal display 100′shown in FIG. 3. Incidentally, display characteristics obtained when thesame root-mean-square voltage is applied to the liquid crystal layers ofthe sub-pixels 10 a and 10 b (i.e., sub-pixel electrodes 18 a and 18 b)in the liquid crystal display 100 according to the embodiment of thepresent invention are approximately equal to those of the conventionalliquid crystal display.

FIG. 4A shows dependence of transmittance on applied voltage when thedisplay is viewed straightly from the front (N1), from the right at anangle of 60 degrees (L1), and from the upper right at an angle of 60degrees (LU1). FIG. 4B is a diagram showing the three transmittances inFIG. 4A after being normalized by taking the transmittance obtained bythe application of the highest grayscale voltage (voltage required todisplay white) as 100%. It shows dependence of normalized transmittanceon applied voltage under the three conditions: a frontal viewingcondition (N2), right side 60-degree viewing condition (L2), andupper-right side 60-degree viewing condition (LU2). Incidentally, thephase “60 degrees” here means an angle of 60 degrees from the normal tothe display surface.

As can be seen from FIG. 4B, frontal viewing display characteristicsdiffer from right side 60-degree viewing and upper-right side 60-degreeviewing display characteristics. This indicates that the γcharacteristics depend on the viewing direction.

FIG. 4C shows differences in the γ characteristics more lucidly. Toillustrate the differences in the γ characteristics clearly, thehorizontal axis represents (frontal normalizedtransmittance÷100)^(1/2.2) while the vertical axis represents grayscalecharacteristics under the N3, L3, and LU3 conditions as follows: frontalviewing grayscale characteristics=(frontal normalizedtransmittance÷100)^(1/2.2), right side 60-degree viewing grayscalecharacteristics=(right side 60-degree normalizedtransmittance÷100)^(1/2.2), and upper-right side 60-degree viewinggrayscale characteristics=(normalized upper-right side 60-degree viewingtransmittance÷100)^(1/2.2), where “^” indicates power and the reciprocalof the power exponent corresponds to a γ value. In a typical liquidcrystal display, the γ value for the frontal viewing grayscalecharacteristics is set at 2.2.

Referring to FIG. 4C, ordinate values coincide with abscissa valuesunder the frontal viewing condition (N3), and thus the grayscalecharacteristics under this condition (N3) are linear. On the other hand,the right side 60-degree viewing grayscale characteristics (L3) andupper-right side 60-degree viewing grayscale characteristics (LU3) arecurvilinear. Deviations of the curves (L3 and LU3) from the straightline under the frontal viewing condition (N3) quantitatively representrespective deviations in the γ characteristics, i.e., deviations(differences) in grayscale display.

The present invention aims at reducing such deviations in normally blackliquid crystal display. Ideally, the curves (L3 and LU3) which representthe right side 60-degree viewing grayscale characteristics (L3) andupper-right side 60-degree viewing grayscale characteristics (LU3)coincide with the straight line which represent the frontal viewinggrayscale characteristics (N3). Effects on improving the γcharacteristics will be evaluated below with reference to a drawingwhich shows deviations in the γ characteristics as is the case with FIG.4C.

With reference FIG. 4B, description will be given of a principle of howthe present invention can reduce the deviations in the γ characteristicsby providing a first sub-pixel and second sub-pixel in each pixel andapplying different root-mean-square voltages to the liquid crystallayers of the sub-pixels. It is assumed here that the first sub-pixeland second sub-pixel have the same area.

With the conventional liquid crystal display 100′, at a voltage at whichthe frontal viewing transmittance is represented by point NA, the rightside 60-degree viewing transmittance is represented by point LArepresenting the right side 60-degree viewing transmittance at the samevoltage as the NA. With the present invention, to obtain the samefrontal viewing transmittance as at point NA, frontal viewingtransmittances of the first sub-pixel and second sub-pixel can be set atpoints NB1 and NB2, respectively. Since the frontal viewingtransmittance at point NB2 is approximately zero and the first sub-pixeland second sub-pixel have the same area, the frontal viewingtransmittance at point NB1 is twice the frontal viewing transmittance atpoint NA. The difference in root-mean-square voltage between points NB1and NB2 is ΔV12. Also, with the present invention, the right side60-degree viewing transmittance is represented by point P, which isgiven as the average of the right side 60-degree viewing transmittancesLB1 and LB2 at the same voltages as at points NB1 and NB2, respectively.

With the liquid crystal display according to the present invention,point P which represents the right side 60-degree viewing transmittanceis closer to point NA which represents the corresponding frontal viewingtransmittance than is point LA which represents the right side 60-degreeviewing transmittance of the conventional liquid crystal display 100′.This means reduced deviations in the γ characteristics.

From the above description, it can be seen that the fact that the rightside 60-degree viewing transmittance (see point LB2) of the secondsub-pixel is approximately zero enhances the effect of the presentinvention. Thus, to enhance the effect of the present invention, it ispreferable to curb increases in transmittance when a black screen isviewed obliquely. From this stand point, it is preferable to install thephase difference compensating elements 20 a and 20 b shown in FIG. 2A,as required, so as to curb increases in transmittance when a blackscreen is viewed obliquely.

The liquid crystal display 100 according to the present inventionimproves the γ characteristics by applying different root-mean-squarevoltages to the two liquid crystal layers of the respective sub-pixels10 a and 10 b in each pixel 10. In so doing, the difference ΔV12 (gk)=V1(gk)−V2 (gk) between the root-mean-square voltages applied to therespective liquid crystal layers of the sub-pixel 10 a and sub-pixel 10b is set in such a way as to satisfy the relationships ΔV12 (gk)>0 voltsand ΔV12 (gk)≧ΔV12 (gk+1). A case in which the above relationships aresatisfied in the entire range of 0<gk≦n will be described below (FIGS.5B and 5C).

FIGS. 5A, 5B, 5C, and 5D show various relationships between theroot-mean-square voltage V1 (gk) applied to the liquid crystal layer ofthe first sub-pixel 10 a and root-mean-square voltage V2 (gk) applied tothe liquid crystal layer of the second sub-pixel 10 b in the pixel 10shown in FIG. 1.

Under voltage application condition A shown in FIG. 5A, the same voltage(V1=V2) is applied to the liquid crystal layers of the two sub-pixels 10a and 10 b. Thus, ΔV12 (gk)=0 volts.

Under voltage condition B shown in FIG. 5B, the relationship V1>V2 holdsand ΔV12 is constant regardless of the value of V1. Thus, under voltagecondition B, the relationship ΔV12 (gk)=ΔV12 (gk+1) is satisfied for anygrayscale gk. This embodiment uses ΔV12 (gk)=1.5 volts as a typicalvalue, but, of course, another value may be used. A large value of ΔV12(gk) enhances the effect of the present invention, but poses a problemof lowered brightness (transmittance) in white mode. Furthermore, thereis the problem that when the value of ΔV12 (gk) exceeds a thresholdvoltage (i.e., Vth shown in FIG. 4B) for the transmittance of the liquidcrystal display, the brightness (transmittance) in black mode increases,lowering display contrast. Thus, it is preferable that ΔV12 (gk)≦Vth.

Under voltage condition C shown in FIG. 5C, the relationship V1>V2 holdsand ΔV12 decreases with increases in V1. Thus, under voltage conditionC, the relationship ΔV12 (gk)>ΔV12 (gk+1) is satisfied for any grayscalegk.

This embodiment uses ΔV12 (0)=1.5 volts and ΔV12 (n)=0 volts as typicalvalues, but, of course, other values may be used. However, as describedabove, it is preferable that ΔV12 (gk)≦Vth from the standpoint ofdisplay contrast during oblique viewing while it is preferable that ΔV12(n)=0 volts from the standpoint of brightness in white mode.

Under voltage condition D shown in FIG. 5D, the relationship V1>V2 holdsand ΔV12 increases with increases in V1. Thus, under voltage conditionD, the relationship ΔV12 (gk)<ΔV12 (gk+1) holds for any grayscale gk.

This embodiment uses ΔV12 (0)=0 volts and ΔV12 (n)=1.5 volts as typicalvalues.

In the liquid crystal display 100 according to the embodiment of thepresent invention, voltage is applied to the liquid crystal layers ofthe sub-pixels 10 a and 10 b such that voltage condition B or voltagecondition C will be satisfied. Incidentally, although the conditionΔV12>0 is satisfied for all grayscales in FIGS. 5B and 5C, ΔV12=0 is allright in the case of an optimum grayscale or the highest grayscale.

Grayscale characteristics of the MVA liquid crystal display undervoltage conditions A to D will be described with reference to FIG. 6.The horizontal axis in FIGS. 6A and 6B represents (frontal normalizedtransmittance÷100)^(1/2.2), the vertical axis in FIG. 6A represents(right side 60-degree normalized transmittance÷100)^(1/2.2), and thevertical axis in FIG. 6B represents (normalized upper-right side60-degree viewing transmittance÷100)^(1/2.2). A straight line whichrepresents frontal viewing grayscale characteristics is shown togetherfor the purpose of reference.

Under voltage condition A, the same voltage (ΔV12 (gk)=0) is applied tothe liquid crystal layers of the sub-pixels 10 a and 10 b. As shown inFIGS. 6A and 6B, the γ characteristics deviate greatly, as with theconventional liquid crystal display shown in FIG. 4.

Voltage condition D has less effect on reducing the viewing angledependence of γ characteristics than do voltage conditions B and C.Voltage condition D corresponds, for example, to voltage conditions forpixel division using conventional capacitance division described inJapanese Patent Laid-Open No. 6-332009. Although it has the effect ofimproving viewing angle characteristics in normally white mode, it doesnot have much effect on reducing the viewing angle dependence of γcharacteristics in normally black mode.

As described above, preferably voltage condition B or C is used toreduce viewing angle dependence of γ characteristics in normally blackmode.

Next, with reference to FIG. 7, description will be given of variationsin white-mode transmittance among voltage conditions, i.e., when thehighest grayscale voltage is applied.

The transmittance in white mode is naturally lower under voltageconditions B and D than under voltage condition A. The transmittance inwhite mode under voltage condition C is equivalent to transmittanceunder voltage condition A. In this respect, voltage condition C ispreferable to voltage conditions B and D. Thus, taking intoconsideration the viewing angle dependence of γ characteristics as wellas transmittance in white mode, it can be said that voltage condition Cis superior.

Next, preferable area ratios between sub-pixels will be described.

According to the present invention, if the root-mean-square voltagesapplied to the liquid crystal layers of the sub-pixels SP1, SP2, . . . ,and SPn are V1, V2, . . . , Vn, if the areas of the sub-pixels are SSP1,SSP2, . . . , and SSPn, and if a relationship V1>V2> . . . >Vn holds,preferably, a relationship SSP1≦SSPn is satisfied. This will bedescribed below.

Assuming that SSP1 and SSP2 are the area of the sub-pixels 10 a and 10 bin the pixel 10 shown in FIG. 1, FIG. 8 compares γ characteristics amongtheir area ratios (SSP1:SSP2)=(1:3), (1:2), (1:1), (2:1), (3:1) undervoltage condition C. FIG. 8A shows right viewing γ characteristics whileFIG. 8B shows upper-right viewing γ characteristics. FIG. 9 showsfrontal viewing transmittance for different split ratios.

As can be seen from FIG. 8, decreasing the area ratio of the sub-pixel(10 a) to which the higher voltage is applied is more effective inreducing the viewing angle dependence of γ characteristics.

The transmittance in white mode takes the maximum value when the arearatio is (SSP1:SSP2)=(1:1) and lowers as the area ratio becomes uneven.This is because a good multi-domain vertical alignment is no longeravailable if the area ratio becomes uneven, reducing the area of thefirst sub-pixel or second sub-pixel. This tendency is pronounced inhigh-resolution liquid crystal displays, which has small pixel areas.Thus, although it is preferable that the area ratio is 1:1, it can beadjusted, as required, taking into consideration its effect on reducingthe viewing angle dependence of γ characteristics, the transmittance inwhite mode, the uses of the liquid crystal display, etc.

Next, the number of pixel divisions will be described.

Although with the liquid crystal display 100 shown in FIG. 1, a pixel 10is composed of two sub-pixels (10 a and 10 b), the present invention isnot limited to this and the number of sub-pixels may be three or more.

FIG. 10 compares the γ characteristics obtained under three conditions:when a pixel is divided into two sub-pixels, when a pixel is dividedinto four sub-pixels, and when a pixel is not divided. FIG. 10A showsright viewing γ characteristics while FIG. 10B shows upper-right viewingγ characteristics. FIG. 11 shows corresponding transmittances of theliquid crystal display in white mode. The area of a pixel was constantand voltage condition B was used.

It can be seen from FIG. 10, increases in the number of sub-pixelsincrease the effect of correcting the deviations in γ characteristics.Compared to when pixels are not divided, the effect is especiallypronounced when a pixel is divided into two sub-pixels. When the numberof divisions is increased from two to four, although there is not muchdifference in deviations in γ characteristics, characteristics areimproved in terms of smooth changes in deviations in relation tograyscale changes. However, as can be seen from FIG. 11, thetransmittance (frontal viewing) in white mode falls as the number ofdivisions increases. It falls greatly, especially when the number ofdivisions is increased from two to four. The main reason for this greatfall is that the area of each sub-pixel is reduced greatly as describedabove. The main reason for reduction in transmittance when no-divisionand two-division conditions are compared is the use of voltage conditionB. Thus, it is advisable to adjust the number of divisions, as required,taking into consideration its effect on reducing the viewing angledependence of γ characteristics, the transmittance in white mode, theuses of the liquid crystal display, etc.

From the above, it can be seen that deviations in γ characteristics,shape distortion of the deviations, and the viewing angle dependence ofγ characteristics are reduced with increases in the number of pixeldivisions. These effects are most pronounced when no-division andtwo-division (two sub-pixels) conditions are compared. Thus, it ispreferable to divides a pixel into two sub-pixels, considering also thefalls in white-mode transmittance resulting from increases in the numberof sub-pixels as well as falls in manufacturability.

In the liquid crystal display 100 shown in FIG. 1, the sub-pixels 10 aand 10 b are connected independently of each other to the TFT 16 a andTFT 16 b, respectively. The source electrodes of the TFTs 16 a and 16 bare connected to the signal lines 14 a and 14 b, respectively. Thus, theliquid crystal display 100 allows any root-mean-square voltage to beapplied to each of the liquid crystal layers of sub-pixels, but requirestwice as many signal lines (14 a and 14 b) as the signal lines 14 of theconventional liquid crystal display 100′ shown in FIG. 3, also requiringtwice as many signal line drive circuits.

In contrast, a liquid crystal display 200 according to another preferredembodiment of the present invention has the same number of signal linesas does the conventional liquid crystal display 100′, but can applymutually different root-mean-square voltages to the liquid crystallayers of the sub-pixels 10 a and 10 b under a voltage condition similarto the voltage condition C described above.

FIG. 12 schematically shows an electrical configuration of the liquidcrystal display 200 according to the other embodiment of the presentinvention. Components which have practically the same functions as thoseof the liquid crystal display 100 shown in FIG. 1 are denoted by thesame reference numerals as the corresponding components and descriptionthereof will be omitted.

A pixel 10 is divided into sub-pixels 10 a and 10 b, which are connectedwith TFT 16 a and TFT 16 b and storage capacitors (CS) 22 a and 22 b,respectively. The TFT 16 a and TFT 16 b have their gate electrodesconnected to a scan line 12, and their source electrodes to the a common(the same) signal line 14. The storage capacitors 22 a and 22 b areconnected to storage capacitor lines (CS bus line) 24 a and 24 b,respectively. The storage capacitors 22 a and 22 b are formed,respectively, by storage capacitor electrodes electrically connectedwith sub-pixel electrodes 18 a and 18 b, storage capacitor counterelectrodes electrically connected with the storage capacitor lines 24 aand 24 b, and insulating layers (not shown) formed between them. Thestorage capacitor counter electrodes of the storage capacitors 22 a and22 b are independent of each other and are supplied with mutuallydifferent storage capacitor counter voltages from the storage capacitorlines 24 a and 24 b.

Next, with reference to drawings, description will be given of aprinciple of how the liquid crystal display 200 can apply differentroot-mean-square voltages to the liquid crystal layers of the sub-pixels10 a and 10 b.

FIG. 13 shows an equivalent circuit for one pixel of the liquid crystaldisplay 200. In the electric equivalent circuit, the liquid crystallayers of the sub-pixels 10 a and 10 b are denoted by 13 a and 13 b.Liquid crystal capacitors formed by the sub-pixel electrodes 18 a and 18b, liquid crystal layers 13 a and 13 b, and counter electrode 17 (commonto the sub-pixels 10 a and 10 b) are denoted by Clca and Clcb.

It is assumed that the liquid crystal capacitors Clca and Clcb have thesame capacitance value CLC (V). The value of CLC (V) depends on theroot-mean-square voltages applied to the liquid crystal layers of thesub-pixels 10 a and 10 b. The storage capacitors 22 a and 22 b connectedto liquid crystal capacitors of the sub-pixels 10 a and 10 bindependently of each other are represented by Ccsa and Ccsb and it isassumed that their capacitance value is CCS.

Both liquid crystal capacitor Clca of the sub-pixel 10 a and storagecapacitor Ccsa have one of their electrodes connected to the drainelectrode of the TFT 16 a provided to drive the sub-pixel 10 a. Theother electrode of the liquid crystal capacitor Clca is connected to thecounter electrode while the other electrode of the storage capacitorCcsa is connected to the storage capacitor line 24 a. Both liquidcrystal capacitor Clcb of the sub-pixel 10 b and storage capacitor Ccsbhave one of their electrodes connected to the drain electrode of the TFT16 b provided to drive the sub-pixel 10 b. The other electrode of theliquid crystal capacitor Clcb is connected to the counter electrodewhile the other electrode of the storage capacitor Ccsb is connected tothe storage capacitor line 24 b. The gate electrodes of the TFT 16 a andTFT 16 b are connected to the scan line 12 and their source electrodesare connected to the signal line 14.

FIG. 14 schematically shows voltage application timings for driving theliquid crystal display 200.

In FIG. 14, the waveform (a) is a voltage waveform Vs of the signal line14, the waveform (b) is a voltage waveform Vcsa of the storage capacitorline 24 a, the waveform (c) is a voltage waveform Vcsb of the storagecapacitor line 24 b, the waveform (d) a voltage waveform Vg of the scanline 12, the waveform (e) is a voltage waveform Vlca of the sub-pixelelectrode 18 a of the sub-pixel 10 a, and the waveform (f) is a voltagewaveform Vlcb of the sub-pixel electrode 18 b of the sub-pixel 10 b. Thebroken lines in the figures indicate a voltage waveform COMMON (Vcom) ofthe counter electrode 17.

Operation of the equivalent circuit in FIG. 13 will be described withreference to FIG. 14.

At time T1, when the voltage Vg changes from VgL to VgH, the TFT 16 aand TFT 16 b are turned on simultaneously and the voltage Vs istransmitted from the signal line 14 to the sub-pixel electrodes 18 a and18 b of the sub-pixels 10 a and 10 b, causing the sub-pixels 10 a and 10b to be charged. Similarly, the storage capacitors Csa and Csb of therespective sub-pixels are charged from the signal line.

At time T2, when the voltage Vg of the scan line 12 changes from VgH toVgL, the TFT 16 a and TFT 16 b are turned off simultaneously.Consequently, the sub-pixels 10 a and 10 b and storage capacitors Csaand Csb are all cut off from the signal line 14. Immediately afterwards,due to drawing effect caused by parasitic capacitance of the TFT 16 aand TFT 16 b and the like, voltages Vlca and Vlcb of the respectivesub-pixels fall by approximately the same voltage Vd to:Vlca=Vs−VdVlcb=Vs−Vd

At this time, the voltages Vcsa and Vcsb of the respective storagecapacitor lines are:Vcsa=Vcom−VadVcsb=Vcom+Vad

At time T3, the voltage Vcsa of the storage capacitor line 24 aconnected to the storage capacitor Csa changes from “Vcom−Vad” to“Vcom+Vad” and the voltage Vcsb of the storage capacitor line 24 bconnected to the storage capacitor Csb changes by twice Vad from“Vcom+Vad” to “Vcom−Vad.” As a result of the voltage changes of thestorage capacitor lines 24 a and 24 b, voltages Vlca and Vlcb of therespective sub-pixels change to:Vlca=Vs−Vd+2×K×VadVlcb=Vs−Vd−2×K×Vadwhere, K=CCS/(CLC (V)+CCS)

At time T4, Vcsa changes from “Vcom+Vad” to “Vcom−Vad” and Vcsb changesfrom “Vcom−Vad” to “Vcom+Vad,” by twice Vad. Consequently, Vlca and Vlcbchange from:Vlca=Vs−Vd+2×K×VadVlcb=Vs−Vd−2×K×VadTo:Vlca=Vs−VdVlcb=Vs−Vd

At time T5, Vcsa changes from “Vcom−Vad” to “Vcom+Vad,” by twice Vad andVcsb changes from “Vcom+Vad” to “Vcom−Vad,” by twice Vad. Consequently,Vlca and Vlcb change from:Vlca=Vs−VdVlcb=Vs−VdTo:Vlca=Vs−Vd+2×K×VadVlcb=Vs−Vd−2×K×Vad

Vcsa, Vcsb, Vlca, and Vlcb alternate the above changes at T4 and T5 atintervals of an integral multiple of horizontal write time 1 H. Themultiple—1, 2, or 3—used for the alternating intervals can be set, asrequired, by taking into consideration a drive method (method ofpolarity inversion, etc.) and display conditions (flickering,graininess, etc.) of the liquid crystal display. These alternatingcycles are repeated until the pixel 10 is rewritten the next time, i.e.,until a time equivalent to T1. Thus, effective values of the voltagesVlca and Vlcb of the sub-pixels are:Vlca=Vs−Vd+K×VadVlcb=Vs−Vd−K×Vad

Thus, the root-mean-square voltages V1 and V2 applied to the liquidcrystal layers 13 a and 13 b of the sub-pixels 10 a and 10 b are:V 1=Vlca−VcomV 2=Vlcb−VcomHence,V 1=Vs−Vd+K×Vad−VcomV 2=Vs−Vd−K×Vad−Vcom

Therefore, difference ΔV12 (=V1−V2) between the root-mean-squarevoltages applied to the liquid crystal layers 13 a and 13 b of thesub-pixels 10 a and 10 b is given as ΔV12=2×K×Vad (where, K=CCS/(CLC(V)+CCS)). This means that mutually different voltages can be applied.

The relationship between V1 and V2 according to this embodiment shown inFIGS. 12 to 14 is shown schematically in FIG. 15.

As can be seen from FIG. 15, in the liquid crystal display 200 accordingto this embodiment, the smaller the value of V1, the larger the value ofΔV12. This is similar to the results obtained under the voltagecondition C described above. The fact that the value of ΔV12 changesdepending on V1 or V2 is attributable to voltage dependence of thecapacitance value CLC (V) of the liquid crystal capacitor.

The γ characteristics of the liquid crystal display 200 according tothis embodiment is shown in FIG. 16. The γ characteristics obtained whenthe same voltage is applied to the sub-pixels 10 a and 10 b are alsoshown in FIG. 16 for comparison. It can be seen from the figure that γcharacteristics are improved also in the liquid crystal displayaccording to this embodiment.

As described above, embodiments of the present invention can improve theγ characteristics of normally black liquid crystal displays, especiallyMVA liquid crystal displays. However, the present invention is notlimited to this and can be applied to IPS liquid crystal displays aswell.

Next, description will be given of liquid crystal displays according toembodiments in a second aspect of the present invention.

Description will be given of a preferred form of a pixel arrangement(array of sub-pixels) or drive method which can reduce “flickering” on aliquid crystal display where each pixel has at least two sub-pixelsdiffering from each other in brightness when displaying an intermediategrayscale. Although configuration and operation of the liquid crystaldisplay according to this embodiment will be described here taking as anexample the liquid crystal display with the divided pixel structureaccording to the embodiment in the first aspect of the presentinvention, the effect produced by a pixel arrangement is not restrictedby a method of pixel division, and a liquid crystal display with anotherdivided-pixel structure may be used as well.

A problem of “flickering” on a liquid crystal display will be describedfirst.

Typical liquid crystal displays are designed to use alternating voltageas the voltage applied to liquid crystal layers of pixels (sometimesreferred to as an “ac driving method”) from a reliability point of view.Magnitude relationship in potential between pixel electrode and counterelectrode is reversed at certain time intervals, and consequently,direction of the electric field (electric lines of force) applied toeach liquid crystal layer is reversed at the time intervals. Withtypical liquid crystal displays in which the counter electrode and pixelelectrode are mounted on different substrates, the direction of theelectric field applied to each liquid crystal layer is reversed from thelight source-to-viewer direction to the viewer-to-light sourcedirection.

Typically, the direction reversal cycle of the electric field applied toeach liquid crystal layer is twice (e.g., 33.333 ms) the frame period(e.g., 16.667 ms). In other words, in a liquid crystal display, thedirection of the electric field applied to each liquid crystal layer isreversed each time a displayed image (frame image) changes. Thus, whendisplaying a still image, if electric field strengths (applied voltages)in alternate directions do not match exactly, i.e., if the electricfield strength changes each time the direction of the electric fieldchanges, the brightness of pixels changes with changes in the electricfield strength, resulting in flickering of the display.

To prevent flickering, it is necessary to equate the electric fieldstrengths (applied voltages) in alternate directions exactly. However,with liquid crystal displays produced industrially, it is difficult toexactly equate the electric field strengths in alternate directions.Therefore, to reduce flickering, pixels with electric fields opposite indirection are placed next to each other, thereby averaging brightness ofpixels spatially. Generally, this method is referred to as “dotinversion” or “line inversion.” Various “inversion driving” methods areavailable, including inversion of a checkered pattern on a pixel bypixel basis (row-by-row, column-by-column polarity inversion: 1-dotinversion), line-by-line inversion (row-by-row inversion: 1-lineinversion), and polarity inversion every two rows and every column. Oneof them is selected as required.

As described above, to implement high quality display, preferably thefollowing three conditions are satisfied: (1) use ac driving so that thedirection of the electric field applied to each liquid crystal layer isreversed at certain time intervals, for example, every frame period, (2)equate the voltages applied to each liquid crystal layer (or quantitiesof electric charge stored in the liquid crystal capacitor) in alternatefield directions as well as quantities of electric charge stored in thestorage capacitor, and (3) place pixels opposite in the direction of theelectric field (sometimes referred to as “voltage polarity”) applied tothe liquid crystal layer, next to each other in each vertical scanningperiod (e.g., frame period). Incidentally, the term “vertical scanningperiod” can be defined as the period after a scan line is selected untilthe scan line is selected again. One scanning period is equivalent toone frame period in the case of non-interlaced driving and correspondsto one field period in the case of interlaced driving. Also, in eachvertical scanning period, the difference (period) between the time whena scan line is selected and the time when the scan line is selectedagain is referred to as one horizontal scanning period (1 H).

The above-described embodiment of the present invention implementsdisplay with excellent viewing angle characteristics by dividing eachpixel into at least two sub-pixels and making their brightness(transmittance) different from each other. The inventor found that wheneach pixel is divided into a plurality of sub-pixels which areintentionally made to vary in brightness, it is preferable that a fourthcondition concerning sub-pixel arrangement is satisfied in addition tothe three conditions described above. Specifically, it is preferablethat the sub-pixels which are intentionally made to vary in brightnessare placed in random order of brightness whenever possible. It is mostpreferable in terms of display quality not to place sub-pixels equal inbrightness next to each other in the column or row direction. In otherword, most preferably sub-pixels equal in brightness are arranged in acheckered pattern.

A drive method, pixel arrangement, and sub-pixel arrangement suitablefor the above-described embodiment of the present invention will bedescribed below.

An example of a drive method for the liquid crystal display according tothe embodiment of the present invention will be described with referenceto FIGS. 17 and 18.

Description will be given below, citing an example in which pixels arearranged in a matrix (rp, cq) with a plurality of rows (1 to rp) andplurality of columns (1 to cq), where each pixel is expressed as P (p,q) (where 1≦p≦rp and 1≦q≦cq) and has at least two sub-pixels SPa (p, q)and SPb (p, q), as shown in FIG. 17. FIG. 17 is a schematic diagrampartially showing a relative arrangement (8 rows×6 columns) of: signallines S-C1, S-C2, S-C3, S-C4, . . . , S-Ccq; scan lines G-L1, G-L2,G-L3, . . . , G-Lrp; storage capacitor lines CS-A and CS-B; pixels P (p,q); and sub-pixels SPa (p, q) and SPb (p, q) which compose the pixels,in the liquid crystal display according to this embodiment.

As shown in FIG. 17, one pixel P (p, q) has sub-pixels SPa (p, q) andSPb (p, q) on either side of the scan line G-Lp which runs through thepixel horizontally at approximately the center. The sub-pixels SPa (p,q) and SPb (p, q) are arranged in the column direction in each pixel.The storage capacitor electrodes (not shown) of the sub-pixels SPa (p,q) and SPb (p, q) are connected to adjacent storage capacitor lines CS-Aand CS-B, respectively. The signal lines S-Ccq which supply signalvoltages to the pixels P (p, q) according to the image displayed runvertically (in the column direction) between pixels to supply the signalvoltages to TFT elements (not shown) of the sub-pixels on the right ofthe signal lines. According to the configuration shown in FIG. 17, onestorage capacitor line or one scan line is shared by two sub-pixels.This has the advantage of increasing the opening rate of pixels.

FIG. 18 shows the waveforms (a)–(j) of various voltages (signals) usedto drive a liquid crystal display with the configuration shown in FIG.17. By driving the liquid crystal display which has the configurationshown in FIG. 17 with voltages which have the voltage waveforms (a)–(j)shown FIG. 18, it is possible to satisfy the four conditions describedabove.

Next, description will be given of how the liquid crystal displayaccording to this embodiment satisfies the four conditions describedabove. For the simplicity of explanation, it is assumed that all pixelsare displaying an intermediate grayscale.

In FIG. 18, the waveform (a) is display signal voltage waveforms (sourcesignal voltage waveforms) supplied to the signal lines S-C1, S-C3, S-C5,. . . (a group of odd-numbered signal lines are sometimes referred to asS-O); the waveform (b) is display signal voltage waveforms supplied tothe signal lines S-C2, S-C4, S-C6, . . . (a group of even-numberedsignal lines are sometimes referred to as S-E); the waveform (c) is astorage capacitor counter voltage waveform supplied to the storagecapacitor line CS-A; the waveform (d) is a storage capacitor countervoltage waveform supplied to CS-B; the waveform (e) is a scan voltagewaveform supplied to the scan line G-L1; the waveform (f) is a scanvoltage waveform supplied to the scan line G-L2; the waveform (g) is ascan voltage waveform supplied to the scan line G-L3; the waveform (h)is a scan voltage waveform supplied to the scan line G-L4; the waveform(i) is a scan voltage waveform supplied to the scan line G-L5; and thewaveform (j) is a scan voltage waveform supplied to the scan line G-L6.The period between the time when the voltage of a scan line changes froma low level (VgL) to a high level (VgH) and the time when the voltage ofthe next scan line changes from VgL to VgH constitutes one horizontalscanning period (1H). The period during which the voltage of a scan lineremains at a high level (VgH) is sometimes referred to as a selectionperiod PS.

Since all pixels are displaying an intermediate grayscale, all displaysignal voltages (waveforms (a) and (b) in FIG. 18) have oscillatingwaveforms of fixed amplitude. Also, the oscillation period of thedisplay signal voltages is two horizontal scanning periods (2 H). Thereason why the display signal voltages are oscillating and the voltagewaveforms of the signal lines S-O (S-C1, S-C3, . . . ) and voltagewaveforms of the signal lines S-E (S-C2, S-C4, . . . ) are 180 degreesout of phase is to satisfy the third condition above. Generally, in TFTdriving, signal line voltages transmitted to a pixel electrode via TFTelements are affected by changes in scan voltage waveforms (sometimescalled a drawing phenomenon). Considering the drawing phenomenon, thecounter voltage is positioned approximately at the center of the signalline voltage waveform after the latter is transmitted to the pixelelectrode. In FIG. 18, where the pixel electrode voltage waveform ishigher than counter voltage, the signal voltage is indicated by a “+”sign and where the pixel electrode voltage waveform is lower thancounter voltage, the signal voltage is indicated by a “−” sign. The “+”and “−” signs correspond to the directions of the electric field appliedto the liquid crystal layers. The directions of the electric field areopposite between when the sign is “+” and when it is “−”.

As described above with reference to FIGS. 12 to 15, when the scanvoltage of a scan line is VgH, the TFT connected to the scan line isturned on, causing the display signal voltage to be supplied to thesub-pixel connected to the TFT. Then, when the scan voltage of the scanline becomes VgL, the storage capacitor counter voltage changes. Sincethe changes (including the direction and sign of the changes) of thestorage capacitor counter voltage differ between the two sub-pixels, sodo the root-mean-square voltages applied to the sub-pixels.

In the example shown in FIG. 18, both oscillation amplitudes and periodsof the storage capacitor counter voltages (waveforms (c) and (d)) takethe same values between the storage capacitor lines CS-A and CS-B: forexample, twice Vad (see FIG. 14) and 1 H, respectively. Also, theoscillating waveforms of CS-A and CS-B will overlap if one of them isphase-shifted 180 degrees. That is, they are 0.5 H out of phase witheach other. An average voltage of each sub-pixel electrode is higherthan the display signal voltage of the corresponding signal lineexisting during the period when the corresponding scan line is in VgHstate if the first voltage change of the corresponding storage capacitorline after the voltage of the corresponding scan line changes from VgHto VgL is an increase, but it is lower than the display signal voltageof the corresponding signal line existing during the period when thecorresponding scan line is in VgH state if the first voltage change ofthe corresponding storage capacitor line is a decrease.

Consequently, if the display signal voltage (waveform (a) or (b)) inFIG. 18 is marked by a “+” sign, the root-mean-square voltage applied tothe liquid crystal layer is higher when the voltage change of thestorage capacitor line is on the rise than when it is on the decline. Onthe other hand, if the display signal voltage (waveform (a) or (b)) inFIG. 18 is marked by a “−” sign, the root-mean-square voltage applied tothe liquid crystal layer is lower when the voltage change of the storagecapacitor line is on the rise than when it is on the decline.

FIG. 17 shows states of the pixels P (p, q) and sub-pixels SPa (p, q)and SPb (p, q) in a vertical scanning period (frame period, in thisexample). The following three symbols shown symmetrically with respectto the scan line of each sub-pixel indicate states of the sub-pixel.

The first symbol H or L indicates the magnitude relationship of theroot-mean-square voltage applied to the sub-pixel, where the symbol Hmeans that the applied root-mean-square voltage is high while the symbolL means that the applied root-mean-square voltage is low. The secondsymbol “+” or “−” indicates the magnitude relationship of voltagesbetween the counter electrode and sub-pixel electrode. In other words,it indicates the directions of the electric field applied to the liquidcrystal layer. The symbol “+” means that the voltage of the sub-pixelelectrode is higher than the voltage of the counter electrode while thesymbol “−” means the voltage of the sub-pixel electrode is lower thanthe voltage of the counter electrode. The third symbol A or B indicateswhether the appropriate storage capacitor line is CS-A or CS-B.

Look at the states of sub-pixels SPa (1, 1) and SPb (1, 1) of the pixelP (1, 1), for example. As can be seen from the waveforms (a) to (e)shown in FIG. 18, during the period when GL-1 is selected (period PS inwhich the scan voltage is VgH), the display signal voltage is “+.” Whenthe scan voltage of GL-1 changes from VgH to VgL, the voltages of thestorage capacitor lines of respective sub-pixels (waveforms (c) and (d))are in the states indicated by the arrows (the first arrows from theleft) shown in FIG. 18. Thus, after the scan voltage of GL-1 changesfrom VgH to VgL, the first voltage change of the storage capacitorcounter voltage of SPa (1, 1) is an increase (indicated by “U” in thewaveform (c)) as shown in FIG. 18. On the other hand, after the scanvoltage of GL-1 changes from VgH to VgL, the first voltage change of thestorage capacitor counter voltage of SPb (1, 1) is a decrease (indicatedby “D” in the waveform (d)) as shown in FIG. 18. Therefore, theroot-mean-square voltage of SPa (1, 1) increases while theroot-mean-square voltage of SPb (1, 1) decreases. Hence, the appliedroot-mean-square voltage of SPa (1, 1) is higher than that of SPb (1,1), and a symbol H is attached to SPa (1, 1) and a symbol L is attachedto SPb (1, 1).

According to the waveform (b) shown in FIG. 18, during the period whenGL-1 is selected, the display signal voltages for SPa (1, 2) and SPb (1,2) of P (1, 2) is “−”. When the scan voltage of GL-1 changes from VgH toVgL, the voltages of the storage capacitor lines of respectivesub-pixels (waveforms (c) and (d)) are in the states indicated by thearrows (the first arrows from the left) shown in FIG. 18. Thus, afterthe scan voltage of GL-1 changes from VgH to VgL, the first voltagechange of the storage capacitor counter voltage of SPa (1, 2) is anincrease (“U”) as shown in FIG. 18. On the other hand, after the scanvoltage of GL-1 changes from VgH to VgL, the first voltage change of thestorage capacitor counter voltage of SPb (1, 2) is a decrease (“D”) asshown in FIG. 18. Therefore, the root-mean-square voltage of SPa (1, 2)decreases while the root-mean-square voltage of SPb (1, 2) increases.Hence, the applied root-mean-square voltage of SPa (1, 2) is lower thanthat of SPb (1, 2), and a symbol L is attached to SPa (1, 2) and asymbol H is attached to SPb (1, 2).

According to the waveform (a) shown in FIG. 18, during the period whenGL-2 is selected, the display signal voltages for (2, 1) and SPb (2, 1)of P (2, 1) is “−”. When the scan voltage of GL-2 changes from VgH toVgL, the voltages of the storage capacitor lines of respectivesub-pixels (waveforms (c) and (d)) are in the states indicated by thearrows (the second arrows from the left) shown in FIG. 18. Thus, afterthe scan voltage of GL-2 changes from VgH to VgL, the first voltagechange of the storage capacitor counter voltage of SPa (2, 1) is adecrease (“D”) as shown in FIG. 18D. On the other hand, after the scanvoltage of GL-2 changes from VgH to VgL, the first voltage change of thestorage capacitor counter voltage of SPb (2, 1) is an increase (“U”) asshown in FIG. 18C. Therefore, the root-mean-square voltage of SPa (2, 1)increases while the root-mean-square voltage of SPb (2, 1) decreases.Hence, the applied root-mean-square voltage of SPa (2, 1) is higher thanthat of SPb (2, 1), and a symbol H is attached to SPa (2, 1) and asymbol L is attached to SPb (2, 1). The states shown in FIG. 17 arebrought about in this way.

The liquid crystal display according to this embodiment can be driven insuch a way as to satisfy the first condition.

Since FIGS. 17 and 18 show states in a frame period, it is not possibleto assess from the figures whether the first condition is satisfied.However, by shifting the phase of the voltage waveform on each signalline (S-O (FIG. 18A) or S-E (FIG. 18B)) by 180 degrees from frame toframe, for example, in FIG. 18, it is possible to implement ac drivingwhere the direction of the electric field applied to each liquid crystallayer is reversed every frame period.

Furthermore, in the liquid crystal display according to this embodiment,to prevent the magnitude relationship of the sub-pixels of the pixels,i.e., the order of brightness of the sub-pixels in a display screen(relative positions of “H” and “L” in FIG. 17) from being changed fromframe to frame, the phase of the voltage waveforms on the storagecapacitor lines CS-A and CS-B is changed by 180 degrees as the phase ofthe voltage waveforms on the signal lines is changed. Consequently, the“+” signs and “−” signs shown in FIG. 17 are inverted in the next frame(for example, (+, H)

(−, H), and (+, L)

(−, L). The first condition described above can be satisfied in thisway.

Now, we will examine whether the second condition is satisfied, i.e.,whether the liquid crystal layer of each sub-pixel (storage capacitor ofthe sub-pixel) is charged to the same level in different fielddirections. In the liquid crystal display according to this embodiment,where different root-mean-square voltages are applied to the liquidcrystal layers of the sub-pixels in each pixel, display quality such asflickering is decisively influenced by sub-pixels ranked high inbrightness, i.e., the sub-pixels indicated by the symbol “H” in FIG. 17.Thus, the second condition is imposed especially on the sub-pixelsindicated by the symbol “H.”

The second condition will be described with reference to voltagewaveforms shown in FIG. 18.

The liquid crystal capacitor and storage capacitor of sub-pixels arecharged during the period when the voltage of the corresponding scanline is VgH (selection period PS). The quantity of electric chargestored in the liquid crystal capacitor depends on the voltage differencebetween the display signal voltage of the signal line and countervoltage (not shown in FIG. 18) during the selection period while thequantity of electric charge stored in the storage capacitor depends onthe voltage difference between the display signal voltage of the signalline and voltage of the storage capacitor line (storage capacitorcounter voltage) during the selection period.

As shown in FIG. 18, the display signal voltage in each selection periodcan be one of the two types indicated by the “+” or “−” sign in thefigures. In either case, there is no voltage change during eachselection period. Regarding the counter voltage (not shown), the same DCvoltage which does not vary with time is applied to all the sub-pixels.

There are two types of storage capacitor line CS-A and CS-B. The voltagewaveform of CS-A is the same during the selection period of any scanline. Similarly, the voltage waveform of CS-B is the same during theselection period of any scan line. In other words, the DC component (DClevel) of the voltage of the storage capacitor lines takes the samevalue during the selection period of any scan line.

Thus, it is possible to satisfy the second condition by adjusting the DCcomponents (DC levels) of the following voltages as required: displaysignal voltage of each scan line, voltage of the counter electrode, andvoltage of each storage capacitor line.

Next, we will verify whether the third condition is satisfied, i.e.,whether pixels opposite in field direction are placed next to each otherin each frame period. In the liquid crystal display according to thisembodiment, where different root-mean-square voltages are applied to theliquid crystal layers of sub-pixels in each pixel, the third conditionapplies to the relationship between the sub-pixels which are suppliedwith the same root-mean-square voltage as well as to the pixels. It isespecially important that the third condition be satisfied by thesub-pixels ranked high in brightness, i.e., the sub-pixels indicated bythe symbol “H” in FIG. 17, as is the case with the second condition.

As shown in FIG. 17, the “+” and “−” symbols which indicate thepolarities (directions of the electric field) of each pixel invert everytwo pixels (two columns) in the row direction (horizontal direction)such as (+, −), (+, −), (+, −), and every two pixels (two rows) in thecolumn direction (vertical direction) such as (+, −), (+, −), (+, −),(+, −). Viewed on a pixel-by-pixel basis, they exhibit a state calleddot inversion, satisfying the third condition.

Next, we will look at the sub-pixels ranked high in brightness, i.e.,the sub-pixels indicated by the symbol “H” in FIG. 17.

Referring to FIG. 17, there is no polarity inversion in the rowdirection as shown, for example, by +H, +H, +H for the sub-pixels SPa inthe first row, but the polarity is inverted every two pixels (two rows)in the column direction as shown, for example, by (+H, −H), (+H, −H),(+H, −H), (+H, −H) in the first column. The state known as lineinversion can be observed at the level of the particularly importantsub-pixels ranked high in brightness, which means that they satisfy thethird condition. The sub-pixels indicated by the symbol L are alsoarranged in a regular pattern, satisfying the third condition.

Next, we will discuss the fourth condition. The fourth conditionrequires that sub-pixels equal in brightness should not be placed nextto each other among the sub-pixels which are intentionally made to varyin brightness.

According to this embodiment, the sub-pixels which are intentionallymade to vary in brightness, i.e., the sub-pixels which have differentroot-mean-square voltages applied to their liquid crystal layersintentionally are indicated by the symbol “H” or “L” in FIG. 17.

In FIG. 17, if sub-pixels are organized into groups of four consistingof two sub-pixels in the row direction and two sub-pixels in the columndirection (e.g., SPa (1, 1), SPb (1, 1), SPa (1, 2), and SPb (1, 2)),the entire matrix is made up of the sub-pixel groups in each of which Hand L are arranged from left to right in the upper row and L and H arearranged in the lower row. Thus, in FIG. 17, the symbols “H” and “L” arearranged in a checkered pattern at the sub-pixel level, satisfying thefourth condition.

Looking at the matrix, at the pixel level, the correspondence betweenthe order of brightness of the sub-pixels in each pixel and position ofthe sub-pixels arranged in the column direction changes in the rowdirection periodically (every pixel) in the case of a pixel in anarbitrary row, but it is constant in the case of a pixel in an arbitrarycolumn. Thus, in a pixel P (p, q) in an arbitrary row, the brightestsub-pixel (sub-pixel indicated by “H,” in this example) is SPa (p, q)when q is an odd number, and SPb (p, q) when q is an even number. Ofcourse, conversely, the brightest sub-pixel may be SPb (p, q) when q isan odd number, and SPa (p, q) when q is an even number. On the otherhand, in a pixel P (p, q) in an arbitrary column, the brightestsub-pixel is always SPa (p, q) or SPb (p, q) in the same columnregardless of whether p is an odd number or even number. The alternativeof SPa (p, q) or SPb (p, q) here means that the brightest sub-pixel isSPa (p, q) in an odd-numbered column regardless of whether p is an oddnumber or even number while it is SPb (p, q) in an even-numbered columnregardless of whether p is an odd number or even number.

As described above with reference to FIGS. 17 and 18, the liquid crystaldisplay according to this embodiment satisfies the four conditionsdescribed above, and thus it can implement high quality display.

Next, a liquid crystal display according to another embodiment using adifferent drive method for pixels and sub-pixels will be described withreference to FIGS. 19 and 20. FIG. 19 and FIG. 20 correspond to FIG. 17and FIG. 18.

As shown in FIG. 20, in the liquid crystal display according to thisembodiment, display signal voltage and storage capacitor counter voltageoscillate every 2 H. Thus the period of oscillation is 4 H (fourhorizontal write times). The oscillations of the signal voltages ofodd-numbered signal lines S-O (S-C1, S-C3, S-C5, . . . ) andeven-numbered signal lines S-E (S-C2, S-C4, S-C6, . . . ) are 180degrees (2 H in terms of time) out of phase with each other. Theoscillations of the voltages of the storage capacitor lines CS-A andCS-B are also 180 degrees (2 H in terms of time) out of phase with eachother. Furthermore, the oscillation of the voltage of the signal lineslags the oscillation of the voltage of the storage capacitor line CS-Aby a phase difference of 45 degrees (⅛ period, i.e., H/2). Incidentally,the phase difference of 45 degrees is used to prevent the VgH-to-VgLvoltage change of the scan line and the voltage change of the storagecapacitor line from overlapping, and the value used here is notrestrictive and another value may be used as required.

With the liquid crystal display according to this embodiment, againevery pixel consists of two sub-pixels which are intentionally made tovary in brightness and are indicated by the symbol “H” or “L.”Furthermore, as shown in FIG. 19, the sub-pixels indicated by the symbol“H” or “L” are arranged in a checkered pattern, which means that thefourth condition is satisfied, as with the above embodiment. Regardingthe first condition, it can be satisfied using the same inversion methodas the one used by the embodiment described with reference to FIGS. 17and 18.

However, the embodiment shown in FIGS. 19 and 20 cannot satisfy thesecond condition described above.

Now, we will look at the brighter sub-pixels Pa (1, 1), Pa (2, 1), Pa(3, 1), and Pa (4, 1) of the pixels P (1, 1), P (2, 1), P (3, 1), and P(4, 1) shown in the first to fourth rows of the first column in FIG. 19.When Pa (1, 1) is being charged, i.e., when G-L1 is selected, thepolarity symbol of the corresponding signal line is “+.” When Pa (3, 1)is being charged, i.e., when G-L3 is selected, the polarity symbol ofthe corresponding signal line is “−”. Also, when Pa (1, 1) is beingcharged, i.e., when G-L1 is selected, the voltage waveform of thecorresponding storage capacitor line CS-A decreases stepwise beginningat approximately the center of the selection period. When Pa (3, 1) isbeing charged, i.e., when G-L3 is selected, the voltage waveform of thecorresponding storage capacitor line CS-A increases stepwise beginningat approximately the center of the selection period. Thus, bycontrolling the phases of the signal voltage waveforms of both storagecapacitor line CS-B and scan line precisely, it is possible to make thestorage capacitor counter electrode have the same DC level both when Pa(1, 1) is being charged and when Pa (3, 1) is being charged. By settingthe DC level to the average between the voltage (equal to the voltage ofthe sub-pixel electrode) of the storage capacitor counter electrode whenPa (1, 1) is being charged and the voltage (equal to the voltage of thesub-pixel electrode) of the storage capacitor counter electrode when Pa(3, 1) is being charged, it is possible to equate the quantities ofelectric charge stored in the storage capacitors of Pa (1, 1) and Pa (3,1). Next, looking at Pa (2, 1), during the corresponding period, i.e.,when G-L2 is selected, the polarity symbol of the corresponding signalline is “−” (the same as with Pa (3, 1) described above) and the voltageof the corresponding storage capacitor line takes a fixed value (not anoscillating waveform such as those above) regardless of time. Thus, byequating the voltage value of the storage capacitor line correspondingto Pa (2, 1) and the DC level described above in relation to Pa (1, 1)and Pa (3, 1), it is possible to equate the quantities of electriccharge stored in the storage capacitors of Pa (1, 1), Pa (3, 1), and Pa(2, 1). However, it is impossible to equate the quantities of electriccharge stored in the storage capacitor Pa (4, 1) with those in thestorage capacitors of Pa (1, 1), Pa (2, 1), and Pa (3, 1) for thefollowing reason. The polarity symbol of the signal line for Pa (4, 1)is the same as that for Pa (1, 1) and the voltage of the correspondingstorage capacitor line takes a fixed value (not an oscillating waveformsuch as those above) regardless of time. Thus, it is necessary to equatethe voltage value (the fixed value described above) of the storagecapacitor line for Pa (4, 1) with the DC level described above inrelation to Pa (1, 1) and Pa (3, 1), as in the case of Pa (2, 1), i.e.,to equate the voltage value (the fixed value described above) of thestorage capacitor line for Pa (4, 1) with that for Pa (2, 1). However,this is not possible because, as can be seen from FIGS. 19 and 20, boththe storage capacitor lines for Pa (2, 1) and Pa (4, 1) are CS-B, whichhas a rectangular oscillating waveform, and the maximum value of theoscillating waveform is selected during the selection period of Pa(2, 1) while the minimum value of the oscillating waveform is selectedduring the selection period of Pa (4, 1), making the two voltagesnecessarily different.

Also, in terms of the third condition to arrange the sub-pixels with thesame polarity so as not to adjoin each other as much as possible, thisembodiment is inferior to the embodiment described with reference toFIGS. 17 and 18.

Referring to FIG. 19, we will look at the polarity inversion of thesub-pixels which have a large voltage applied to their liquid crystallayers intentionally, i.e., the sub-pixels indicated by the symbol H,out of the sub-pixels composing pixels. In FIG. 19, there is no polarityinversion in the row direction as shown, for example, by +H, +H, +H forthe sub-pixels SPa in the first row (as with FIG. 17), but the polarityis inverted every four pixels in the column direction as shown, forexample, by (+H, −H, −H, +H), (+H, −H, −H, +H) in the first column. Inthe embodiment described with reference to FIGS. 17 and 18, polarityinversion occurs every two pixels, ½ the polarity inversion cycle ofthis embodiment. In other words, in the embodiment described withreference to FIGS. 17 and 18, polarity inversion occurs twice asfrequently as in this embodiment described with reference to FIGS. 19and 20. In this respect, this embodiment (described with reference toFIGS. 19 and 20) is inferior to the embodiment described with referenceto FIGS. 17 and 18.

Display quality was actually compared between the drive method of theprevious embodiment which implements the pixel arrangement shown in FIG.17 and the drive method of this embodiment and differences were observedin the display quality. Specifically, when, for example, a64/255-grayscale display which produces relatively large brightnessdifferences among sub-pixels which were intentionally made to vary inbrightness was observed with the line of sight fixed, no significantdifference was observed between the two drive methods. However, when thedisplay was observed by moving the line of sight, horizontal streakswere observed in the case of the drive method of this embodiment (FIG.19) whereas the drive method of the previous embodiment (FIG. 17) wasfree of such a problem. It is believed that the difference was caused bythe difference in the polarity inversion cycle described above. Sincethe brighter of the two sub-pixels contained in each pixel is moreconspicuous, it is preferable to minimize the polarity inversion cycleof the brighter sub-pixel. Each pixel is divided into two sub-pixels inthe example described above, but if it is divided into three or moresub-pixels, it is preferable to arrange them in such a way as tominimize the polarity inversion cycle of the brightest sub-pixel.Needless to say, it is most preferable that all the other sub-pixelshave the same polarity inversion cycle as the brightest sub-pixel.

Next, with reference to FIGS. 21A and 21B, description will be given ofan embodiment which makes the above-described horizontal streaks moreinconspicuous using a shorter polarity inversion cycle than theembodiment shown in FIG. 17 even if the display is observed by movingthe line of sight.

According to the embodiment shown in FIG. 17, although the “+” and “−”signs of the brighter sub-pixels (indicated by the symbol “H”) composingpixels are inverted in the column direction as shown by (+, −), (+, −),(+, −), (+, −), they are not inverted in the row direction as shown by+, +, +, +, +, + or −, −, −, −, −, −. In contrast, according to theembodiment shown in FIG. 21, the “+” and “−” signs of the brightersub-pixels are inverted not only in the column direction as shown by (+,−), (+, −), (+, −), (+, −), but also in the row direction as shown by(+, −), (+, −) Thus, this embodiment shown in FIG. 20 uses a shorterpolarity inversion cycle than the embodiment shown in FIG. 17. In thisrespect, this embodiment shown in FIG. 20 is more preferable than theembodiment shown in FIG. 17.

Even in the embodiment shown in FIG. 21, out of the sub-pixels composingthe pixels, the brighter sub-pixels indicated by the symbol “H” arearranged in a checkered pattern, satisfying the fourth condition.

The pixel arrangement shown in FIG. 21A can be implemented, for example,as follows.

As shown schematically in FIG. 21B, the storage capacitor counterelectrodes for the sub-pixels in each row are connected alternately tothe storage capacitor line CS-A or CS-B every two columns. Thisstructural change can be seen clearly by comparing FIG. 21 for thisembodiment and FIG. 17 or 18 for the embodiment described earlier.Specifically, this can be seen by looking at the storage capacitor linesselected at the sub-pixel in the row direction. For example, in the rowof sub-pixels SPa (1, 1) to SPa (1, 6), out of the storage capacitorcounter electrodes indicated by the symbol “A” or “B,” “A” is selectedfor SPa (1, 1), “B” for SPa (1, 2) and SPa (1, 2), “A” for SPa (1, 4)and SPa (1, 5), and “B” for SPa (1, 6) in FIG. 21 (this embodiment)whereas “A” is selected for all the sub-pixels SPa (1, 1) to SPa (1, 6)in FIG. 17 or 18 (the embodiment described earlier).

The voltage waveforms (a)–(j) shown in FIG. 18 can be used as thevoltage waveforms supplied to the lines, including the storage capacitorlines CS-A and CS-B, according to this embodiment shown in FIG. 21.However, since display signal voltages are inverted every two columns,the display signal voltages having the waveform (a) shown in FIG. 18 aresupplied to S-C1, S-C2, S-C5, S-C6, . . . shown in FIG. 21A, while thedisplay signal voltages having the waveform (b) shown in FIG. 20 aresupplied to S-C3, S-C4, S-C7 (not shown), S-C8 (not shown), . . . inFIG. 21A.

Although in the embodiments described above, the storage capacitorcounter voltages supplied to the storage capacitor lines are oscillatingvoltages which have rectangular waveforms with a duty ratio of 1:1, thepresent invention can also use rectangular waves with a duty ratio ofother than 1:1. Besides other waveforms such as sine waves or triangularwaves may also be used. In that case, when TFTs connected to a pluralityof sub-pixels are turned off, the changes which occur in the voltagessupplied to the storage capacitor counter electrodes of sub-pixels canbe varied depending on the sub-pixels. However, the use of rectangularwaves makes it easy to equate quantities of electric charge stored indifferent sub-pixels (liquid crystal capacitors and storage capacitors)as well as root-mean-square voltages applied to different sub-pixels.

Also, although in the embodiments described above with reference toFIGS. 17 and 21, the oscillation period of the oscillating voltagessupplied to the storage capacitor lines (waveforms (c) and (d)) are 1 Has shown in FIG. 18, it may be a fraction of 1 H, such as 1/1 H, ½ H, ⅓H, ¼ H, etc., obtained by dividing 1 H by a natural number. However, asthe oscillation period of the oscillating voltages becomes shorter, itbecomes difficult to build drive circuits or power consumption of thedrive circuits increases.

As described above, the first aspect of the present invention can reducethe viewing angle dependence of γ characteristics in a normally blackliquid crystal display. In particular, it can achieve extremely highdisplay quality by improving γ characteristics of liquid crystaldisplays with a wide viewing angle such as MVA or ASV liquid crystaldisplays.

The second aspect of the present invention can reduce flickering on aliquid crystal display driven by alternating voltage. By combining thefirst and second aspects of the present invention it is possible toprovide a normally black liquid crystal display with reduced flickering,improved viewing angle characteristics, and high quality display.

1. A liquid crystal display, comprising a plurality of pixels each ofwhich has a liquid crystal layer and a plurality of electrodes forapplying an electric field to the liquid crystal layer, wherein: theplurality of pixels are arranged in a matrix (rp, cq) with a pluralityof rows (1 to rp) and plurality of columns (1 to cq) and each pixel isexpressed as P (p, q), where 1≦p≦rp and 1≦q≦cq; each of the plurality ofpixels has at least two sub-pixels SPa (p, q), SPb (p, q), etc. arrangedin the column direction; and the at least two sub-pixels differ fromeach other in brightness when displaying an intermediate grayscale, andthe position of the brightest of the at least two sub-pixels arranged inthe column direction changes periodically in the row direction in thecase of a pixel in an arbitrary row, but it is constant in the case of apixel in an arbitrary column.
 2. The liquid crystal display according toclaim 1, wherein in each of the plurality of pixels, correspondencebetween the order of brightness of the at least two sub-pixels andposition of the at least two sub-pixels arranged in the column directionchanges in the row direction periodically in the case of a pixel in anarbitrary row, but it is constant in the case of a pixel in an arbitrarycolumn.
 3. The liquid crystal display according to claim 1, wherein whendisplaying an intermediate grayscale, the at least two sub-pixels ineach of the plurality of pixels are placed such that sub-pixels equal inthe order of brightness will not adjoin each other.
 4. The liquidcrystal display according to claim 1, wherein the at least twosub-pixels are SPa (p, q) and SPb (p, q), and when displaying anintermediate grayscale: the brightest sub-pixel in a pixel P (p, q) inan arbitrary row is SPa (p, q) in the case where q is an odd number andSPb (p, q) in the case where q is an even number, or SPb (p, q) in thecase where q is an odd number and SPa (p, q) in the case where q is aneven number; and the brightest sub-pixel in a pixel P (p, q) in anarbitrary column is either SPa (p, q) or SPb (p, q) regardless ofwhether p is an odd number.
 5. The liquid crystal display according toclaim 4, wherein when displaying an intermediate grayscale, the at leasttwo sub-pixels in each of the plurality of pixels are placed such thatthe brightest sub-pixels form a checkered pattern.
 6. The liquid crystaldisplay according to claim 1, wherein: direction of the electric fieldapplied to the liquid crystal layers in the plurality of pixels isreversed every vertical scanning period; and when displaying anintermediate grayscale, the direction of the electric field is reversedperiodically in the row direction in the case of pixels in an arbitraryrow and it is reversed every pixel in the column direction in the caseof pixels in an arbitrary column.
 7. The liquid crystal displayaccording to claim 6, wherein the direction of the electric field isreversed every pixel in the row direction in the case of pixels in anarbitrary row.
 8. The liquid crystal display according to claim 6,wherein the direction of the electric field is reversed every two pixelsin the row direction in the case of pixels in an arbitrary row.
 9. Theliquid crystal display according to claim 1, wherein: the liquid crystaldisplay operates in normally black mode; the at least two sub-pixelsinclude two sub-pixels SPa (p, q) and SPb (p, q); and when each of theplurality of pixels displays a grayscale gk which satisfies 0≦gk≦n,where gk and n are integers not less than zero and a larger value of gkcorresponds to higher brightness, relationships ΔV12 (gk)>0 volts andΔV12 (gk)≧ΔV12 (gk+1) are satisfied at least in a range 0<gk≦n−1 if itis assumed that ΔV12 (gk)=V1 (gk)−V2 (gk), where V1 (gk) and V2 (gk) areroot-mean-square voltages applied to the liquid crystal layers of thefirst sub-pixel and the second sub-pixel, respectively.
 10. The liquidcrystal display according to claim 9, wherein a relationship ΔV12(gk)≧ΔV12 (gk+1) is satisfied at least in a range 0<gk≦n−1.
 11. Theliquid crystal display according to claim 9, wherein: SPa (p, q) and SPb(p, q) each comprise: a liquid crystal capacitor formed by a counterelectrode and a sub-pixel electrode opposing the counter electrode viathe liquid crystal layer, and a storage capacitor formed by a storagecapacitor electrode connected electrically to the sub-pixel electrode,an insulating layer, and a storage capacitor counter electrode opposingthe storage capacitor electrode via the insulating layer; and thecounter electrode is a single electrode shared by SPa (p, q) and SPb (p,q), and the storage capacitor counter electrodes of SPa (p, q) and SPb(p, q) are electrically independent of each other.
 12. The liquidcrystal display according to claim 11, comprising two switching elementsprovided for SPa (p, q) and SPb (p, q), respectively, wherein the twoswitching elements are turned on and off by scan line signal voltagessupplied to a common scan line; display signal voltages are applied tothe respective sub-pixel electrodes and storage capacitor electrodes ofSPa (p, q) and SPb (p, q) from a common signal line when the twoswitching elements are on; voltages of the respective storage capacitorcounter electrodes of SPa (p, q) and SPb (p, q) change after the twoswitching elements are turned off; and the amounts of change defined bythe direction and magnitude of the change differ between SPa (p, q) andSPb (p, q).
 13. The liquid crystal display according to claim 12,wherein the changes in the voltages of the storage capacitor counterelectrodes of SPa (p, q) and SPb (p, q) are equal in amount and oppositein direction.
 14. The liquid crystal display according to claim 12,wherein the voltages of the storage capacitor counter electrodes of SPa(p, q) and SPb (p, q) are oscillating voltages 180 degrees out of phasewith each other.
 15. The liquid crystal display according to claim 14,wherein the oscillating voltages of the storage capacitor counterelectrodes of SPa (p, q) and SPb (p, q) each have a period approximatelyequal to one horizontal scanning period.
 16. The liquid crystal displayaccording to claim 14, wherein the oscillating voltages of the storagecapacitor counter electrodes of SPa (p, q) and SPb (p, q) each have aperiod shorter than one horizontal scanning period.
 17. The liquidcrystal display according to claim 14, wherein the oscillating voltagesof the storage capacitor counter electrodes of SPa (p, q) and SPb (p, q)are approximately equal within any horizontal scanning period ifaveraged over the period.
 18. The liquid crystal display according toclaim 16, wherein the period of the oscillation is one-half of onehorizontal scanning period.
 19. The liquid crystal display according toclaim 14, wherein the oscillating voltages are rectangular waves with aduty ratio of 1:1.
 20. The liquid crystal display according to claim 1,wherein SPa (p, q) and SPb (p, q) have different areas, of which thesmaller area belongs to SPa (p, q) or SPb (p, q) whichever has a largerroot-mean-square voltage applied to its liquid crystal layer.
 21. Theliquid crystal display according to claim 1, wherein the area of SPa (p,q) and area of SPb (p, q) are practically equal.